Dense fog is threatening to cause travel disruption across southern England, with thousands of air passengers facing possible delays.
The Met Office has issued a severe weather warning for London, south-east England and parts of south-west England over dense fog that is likely to hit the morning rush hour.
Heathrow airport warned passengers to check their flight details before they leave home as visibility at the hub was reduced to 100 yards.
A yellow warning of fog is in place until 10.30am, but freezing temperatures and low winds could see it persist into the afternoon, Met Office forecaster Mark Wilson said.
"Heathrow is currently on 100 metres visibility which is very dense fog, Gatwick is on 100 metres, London City Airport is on 100 metres and Stansted is on 100 metres," he said.
"It looks like it will generally improve through the last part of the morning and into the afternoon, but one or two spots will hang on to fog for much of the rest of today."
Road users are also being warned of difficult driving conditions after freezing conditions overnight.
Temperatures dipped to their lowest in Katesbridge, Northern Ireland, where minus 6.9C (20F) was recorded.
In England a low of minus 6.2C (21F) was recorded in Northolt, around 10 miles from Heathrow.
A message on the airport's website said: "Foggy weather across London is expected on Monday 23 January and some passengers may experience disruption to their journey as a result.
"Heathrow advises passengers to check their flight status with their airline before coming to the airport.
"Heathrow's top priority is the safety of passengers and we apologise to those whose travel has been affected."
Fog is expected to cause some disruption to flights at Heathrow tomorrow (Jan 23). Please check with your airline prior to travelling.— Heathrow Airport (@HeathrowAirport) January 22, 2017
Mr Wilson said temperatures will struggle to top 0C (32F) in the worst affected areas on Monday, where fog is likely to persist.
Elsewhere highs of between 3C (37F) and 5C (41F) are expected.
A Stansted Airport spokeswoman said there had not been any cancellations, but advised passengers to check with their airlines for the latest information and allow enough time to get to the airport.